Sic epitaxial wafer and method for manufacturing the same

ABSTRACT

A SiC epitaxial wafer including: a SiC epitaxial layer that is formed on a SiC substrate having an off angle, wherein the surface density of triangular defects, in which a distance from a starting point to an opposite side in a horizontal direction is equal to or greater than (a thickness of the SiC epitaxial layer/tan(x))×90% and equal to or less than (the thickness of the SiC epitaxial layer/tan(x))×110%, in the SiC epitaxial layer is in the range of 0.05 pieces/cm 2  to 0.5 pieces/cm 2  (where x indicates the off angle).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Rule 53(b) Divisional of U.S. application Ser. No.14/407,397 filed Dec. 11, 2014, which is a National Stage ofInternational Application No. PCT/JP2013/066808 filed Jun. 19, 2013,which claims benefit from Japanese Patent Application No. 2012-137912filed Jun. 19, 2012, the above-noted applications incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a SiC epitaxial wafer and a method ofmanufacturing the same.

BACKGROUND ART

Silicon carbide (SiC) is characterized in that a breakdown electricfield is one digit greater than that of silicon (Si), and a band gap andthermal conductivity are about three times more than those of Si.Therefore, silicon carbide (SiC) is expected to be applied to, forexample, power devices, high-frequency devices, and high-temperatureoperation devices.

A SiC epitaxial wafer is manufactured by growing a SiC epitaxial filmserving as an active region of a SiC semiconductor device on a SiCsingle crystal substrate, in general, using a chemical vapor deposition(CVD) method, in which the SiC single crystal substrate is processedfrom a SiC bulk single crystal produced by, for example, a sublimationmethod.

A defect with a triangular shape (hereinafter referred to as a“triangular defect”) has been known as a defect in the SiC epitaxialfilm. The triangular defect is formed in a direction in which the apexof a triangle and an opposite side (base) are sequentially arranged in aline along a step-flow growth direction (NPL 3). That is, the oppositeside (base) of the triangular defect is disposed in a directionperpendicular to the <11-20> direction. A plurality of origins of thetriangular defect are considered. Examples of the origins includedamage, such as a polishing flaw which remains on the surface of asubstrate (wafer) (PTL 1), a 2-dimensional nucleus which is formed in aterrace during step-flow growth (PTL 2), a different polytype of crystalnucleus which is formed at the interface between the substrate and theepitaxial film in an oversaturated state at the early stage of growth(NPL 1), and a defect which has a minute broken piece of a SiC film,which will be described below, as a starting point. The triangulardefect is grown as the SiC epitaxial film is grown. That is, thetriangular defect is grown with step-flow growth such that its areaincreases while a shape which is substantially similar to a triangle andhas the starting point as its apex is maintained (see the schematicdiagram of FIG. 5). Therefore, in general, in as early stage of thegrowth of the SiC epitaxial film as the starting point of the defectoccurs, the triangular defect grows large, and the depth of the startingpoint in the film can be estimated from the size of the triangulardefect.

A reduction in the triangular defects is indispensable in order toimprove yield in the mass production of the SiC epitaxial wafer. PTL 1and PTL 2 disclose methods of reducing the triangular defects.

In addition, as a cause of deterioration in the quality of the SiCepitaxial film, there is a minute broken piece (hereinafter referred toas a “downfall”) of a SiC film which falls on a SiC wafer, in a SiCepitaxial film, or on the SiC epitaxial film. The downfall is a piece ofa SiC film which peels off from the SiC film deposited on a ceiling thatis provided on the upper side of the chamber so as to face the uppersurface of a susceptor including a wafer mounting portion on which a SiCwafer (SiC substrate) is placed. The downfall can be the starting pointof the triangular defect.

Here, when the SiC epitaxial film is grown, it is necessary to heat theSiC wafer, which is a substrate, at a high temperature and to maintainthe temperature. As a method of heating the substrate and maintainingthe temperature, a method has been mainly used which performs heatingusing heating means that is located below the susceptor and/or above theceiling (see PTL 3, NPL 2, and NPL 3). When the ceiling is heated, it isgenerally heated by high-frequency induction heating using an inductioncoil. The ceiling which is made of carbon suitable for thehigh-frequency induction heating is generally used.

While the SiC epitaxial film is formed, SiC is deposited not only on theSiC wafer but also on the ceiling or other members which are placed in achamber (SiC-CVD furnace). When the film is repeatedly formed, theamount of SiC deposited on, for example, the ceiling, increases.Therefore, the problem of the downfall becomes notable particularly inmass production.

A reduction in the downfall is indispensable in order to improve yieldin the mass production of the SiC epitaxial wafer.

However, in the SiC epitaxial wafer, nitrogen also becomes a dopant.Therefore, when the SiC epitaxial wafer is manufactured, a chamber(SiC-CVD furnace) 200 is arranged in a glove box 100, as shown in FIG.23. Then, the glove box is filled with inert gas, such as argon gas. Ingeneral, a film forming process which forms a SiC epitaxial film on aSiC wafer while circulating the inert gas in the glove box through afilter 300 provided in the glove box (turning on circulation) isperformed to manufacture the SiC epitaxial wafer. An arrow connectingtwo filters 300 schematically indicates the circulation of the inertgas.

As the filter which is provided in the glove box, for example, a filteris used which has the minimum removal ratio (particle collection ratio)with respect to particles with a size of 0.3 μm and has a particleremoval ratio of 99.97% or more with respect to particles with a size of0.3 μm.

Whenever a cover 201 of the chamber (SiC-CVD furnace) is opened in orderto place the SiC substrate or to withdraw the manufactured SiC epitaxialwafer, a deposit (particle), such as SiC, which is attached to themembers which are placed in the chamber is scattered in the glove boxand contaminates the inside of the glove box. The main purpose of thecirculation of the inert gas in the glove box through the filter is toremove the deposit.

In FIG. 23, a dotted line above the cover which is represented byreference numeral 201 indicates a state in which the cover is opened anda vertical arrow indicates that the cover can be opened and closed.

CITATION LIST Patent Literature

-   [PTL 1]: Japanese Patent No. 4581081-   [PTL 2]: Japanese Unexamined Patent Application, First Publication    No. 2009-256138-   [PTL 3]: Published Japanese Translation No. 2004-507897 of the PCT    International Publication-   [PTL 4]: Japanese Unexamined Patent Application, First Publication    No. 2009-164162-   [PTL 5]: Japanese Patent No. 4959763

Non-Patent Literature

-   [NPL 1]: Journal of Applied Physics 105 (2009) 074513-   [NPL 2]: Materials Science Forum Vols. 483-485 (2005) pp. 141-146-   [NPL 3]: Materials Science Forum Vols. 556-557 (2007) pp. 57-60

SUMMARY OF INVENTION Problems to be Solved by the Invention

Here, when the SiC epitaxial wafer is manufactured, first, a substrateplacement operation of placing the SiC wafer (SiC (single crystal)substrate) in the chamber needs to be performed. In the related art, thesubstrate placement operation (that is, the operation which is performedwhile the cover of the chamber is opened) is performed with thecirculation turned off. The reason why the substrate placement operationis performed, with the circulation turned off, is as follows. When thecirculation is turned on, the amount of movement of gas in the glove boxis large and a deposit, such as a SiC deposit attached to the inner wallof the chamber or members which are placed in the chamber, is scattered.The scattered deposit is attached to the SiC wafer (SiC substrate)during the placement operation.

Particles including the SiC deposit and other deposits are attached tothe SiC wafer (SiC substrate) in various stages. For example, thefollowing situations are considered: particles are attached when thecover of the chamber (SiC-CVD furnace) is closed; particles are attachedwhen a raw material gas is introduced; and particles are attached when atemperature increases for an epitaxial growth. These correspond to casesin which particles are attached in the chamber (SiC-CVD furnace). It isnot easy to solve the problem that particles are attached in thechamber. In addition, problems related to an environment in the glovebox, which is an environment outside the chamber, have not been examinedwell. For example, the following have not been examined: therelationship between the number of floating particles and defects of theSiC epitaxial wafer caused by the particles; and methods of reducing thedefects. When the particles in the glove box are also attached onto theSiC substrate, they cause triangular defects.

However, it is difficult to sufficiently reduce the density oftriangular defects other than the triangular defects caused by theexisting particles in the glove box, using the methods disclosed in PTL1 and PTL 2. One of the reasons is the presence of other triangulardefects, the cause of which has not been well known.

Regarding the downfall, the downfall can be prevented from falling fromthe ceiling on the SiC substrate or the SiC epitaxial film grown on theSiC substrate, by using the method disclosed in PTL 4. However, it isdifficult to suppress the deposition of SiC (or the growth of a SiCfilm) on the ceiling which causes the downfall. For this reason, it isnecessary to clean the ceiling. However, when cleaning is performed,there is a problem that the operation rate of the apparatus is reduced.In addition, SiC is deposited on the lower surface of a cover plate.Therefore, when the film is repeatedly formed, there is a problem thatthe downfall falls from the cover plate.

The invention has been made in view of the above-mentioned problems andan object of the invention is to provide a SiC epitaxial wafer with alow surface density of triangular defects which include particlesattached to a SiC substrate as starting points and a method ofmanufacturing the same. Another object of the invention is to provide aSiC epitaxial wafer with a low surface density of triangular defectswhich include a piece of a member placed in a chamber as a startingpoint and a method of manufacturing the same.

Means for Solving the Problem

The inventors made a thorough study on the relationship between the onor off state of circulation, and the number of existing particles bysize in the glove box and the total number of the existing particles andthe relationship between the on or off state of circulation and thesurface density of triangular defects formed in the epitaxial film ofthe SiC epitaxial wafer. The inventors obtained the results whichoverturned common sense. That is, the inventors found that, when a SiCwafer (SiC substrate) substrate placement operation was performed withcirculation maintained in an on state rather than in an off state, thesurface density of triangular defects caused by the existing particlesin the glove box was low.

The inventors found a triangular defect having a piece of a member whichis placed in a chamber (SiC-CVD furnace) as a starting point. That is,the inventors found that the piece of the member which is placed in thechamber fell on a SiC substrate or a SiC epitaxial film during growthdue to a certain cause and a new type of triangular defect was grownfrom the piece as a starting point. In the related art, as describedabove, for example, a polishing flaw which remains on the surface of thesubstrate (wafer) or a different polytype of crystal nucleus which isgenerated on a step have been known as the starting point of thetriangular defect. However, the currently found triangular defect hasthe piece of the member which is placed in the chamber as the startingpoint.

Here, the triangular defect which is caused by existing particles in theglove box includes, as the starting point, a particle which is attachedonto the SiC substrate when the SiC substrate is placed in the SiC-CVDfurnace before epitaxial growth. The distance of the triangular defectfrom the starting point to the opposite side (base) in the horizontaldirection (distance in a plan view) is determined by the off angle ofthe SiC substrate and the thickness of the epitaxial layer. Therefore,it is possible to determine whether the triangular defect is likely tobe caused by the existing particles in the glove box on the basis of thedistance of the triangular defect from the starting point to theopposite side in the horizontal direction (distance in a plan view).

The triangular defect including the piece of the member which is placedin the chamber as the starting point is characterized in that thestarting point is unclear in an optical microscope image or an image(hereinafter, referred to as a candela image) obtained by an opticalsurface inspection apparatus using a laser beam, which will be describedbelow. The characteristics show that the triangular defect is likely tobe caused by the piece of the member which is placed in the chamber.

(Particles in Glove Box)

FIG. 1 is a graph showing the relationship between the number ofparticles in the glove box which is measured by a particle counterbefore a SiC substrate is placed in the SiC-CVD furnace and the surfacedensity of triangular defects which is measured from a SiC epitaxiallayer (a thickness of 12.5 μm) of a SiC epitaxial wafer after the SiCepitaxial wafer having the SiC epitaxial layer on the SiC substrate ismanufactured. The horizontal axis indicates the number of existingparticles in the glove box and the vertical axis indicates the measuredsurface density (pieces/cm²) of the triangular defects. The number ofparticles in the glove box is the number of particles in 28.8 liters (1ft³ (cubic feet)) of gas.

Hot Wall SiC CVD (VP2400HW) manufactured by Aixtron Corporation was usedas the glove box and the SiC-CVD furnace (hereinafter, the results ofthe invention were obtained using the same). The gas which wascirculated was argon and the filter was a HEPA filter. The removal ratio(particle collection ratio) of the filter varies depending on a particlesize. The filter used had a minimum removal ratio (particle collectionratio) with particles with a size of 0.3 μm and had a particle removalratio of 99.97% or more with respect to particles with a size of about0.3 μm (hereinafter, the results of the invention were obtained usingthe same). Aero Track 9110 manufactured by Nitta Corporation was used asthe particle counter (hereinafter, the results of the invention wereobtained using the same).

As shown in FIG. 1, as the number of existing particles in the glove boxincreases, the surface density of the triangular defects in the SiCepitaxial layer increases monotonously.

Here, the measured triangular defects include triangular defects causedby the particles in the glove box and triangular defects caused byparticles other than the particles in the glove box (mainly a depositwhich is attached to a member which is placed in the SiC-CVD furnace ora piece which peels off from the member (a piece of the member)). InFIG. 1, the triangular defects, which are present even though the numberof particles is close to zero, are caused by particles other than theexisting particles in the glove box.

FIG. 2 is a graph showing the measurement result of the number ofparticles in the glove box when the circulation (gas circulation) isalternatively and repeatedly turned on and off. In the graph, thehorizontal axis indicates time (minute), the vertical axis indicates thenumber of particles in 28.8 liters of gas, and vertical dashed linesindicate the switching between the on and off states of circulation. Inaddition, in FIG. 2, “*” indicates that a simulated operation(corresponding to the SiC substrate placement operation) is performed byhands in the glove box at that time.

As can be seen from FIG. 2, when the circulation is turned on, thenumber of particles in the glove box is significantly less than thatwhen the circulation is turned off. In addition, when the circulation isswitched from the off state to the on state, the number of particles inthe glove box is reduced to the minimum value within a few minutes andthe effect of the circulation is sufficient.

When the circulation is turned off, the number of particles in the glovebox increases. It is considered that this is because the particles inthe glove box are scattered due to the backward flow of the particlesfrom the filter or a disturbance in the flow of gas and are not removedby the filter since the circulation is in the off state.

As can be seen from this result, even when the circulation is turned onto clean the inside of the glove box and is then turned off, the glovebox is contaminated.

FIG. 3 is a graph showing the relationship between the number ofparticles in the glove box, and the opening and closing of the cover ofthe SiC-CVD furnace (chamber) accompanied by the SiC substrate placementoperation and the on or off state of the circulation (gas circulation).

In FIG. 3, “♦” (Example) indicates an example in which the circulationis turned on through the opening and closing of the cover of the SiC-CVDfurnace (chamber) accompanied by the SiC substrate placement operation.In addition, “Δ” (Comparative Example 1) and “□” (Comparative Example 2)indicate examples in which, when the cover of the SiC-CVD furnace(chamber) is opened, the circulation is changed from an on state to anoff state, the SiC substrate is placed, the cover is closed, and thecirculation is turned on. In the graph, the horizontal axis indicatestime (minute) and the vertical axis indicates the number of particles in28.8 liters of gas. In FIG. 3, a vertical line represented by “chargestart” indicates the time when the cover is opened in order to place theSiC substrate. In addition, in each example, a vertical line after thevertical line represented by “charge start” indicates the time when theplacement of the SiC substrate is completed and the cover is closed. Ineach example, a circle after the vertical line indicates the number ofparticles immediately before the formation of the epitaxial film starts.

As shown in FIG. 3, when the circulation is maintained in the on state(“♦”), there is little increase in the number of particles in the glovebox even though the cover of the SiC-CVD furnace (chamber) is opened andthe SiC substrate placement operation is performed.

In contrast, in the examples (“Δ” and “□”) in which the circulation ischanged from the on state to the off state, the cover of the SiC-CVDfurnace (chamber) is opened, and the SiC substrate placement operationis performed, the number of particles in the glove box increases.

It is considered that the increase in the number of particles is causedby the backward flow of the particles from the filter and the scatteringof the particles in the SiC-CVD furnace (chamber).

Table 1 shows the measurement results of the surface density(pieces/cm²) of all triangular defects in the epitaxial layer of the SiCepitaxial wafer, the surface density (pieces/cm²) of triangular defectswith a width (a distance from the starting point to the opposite side ofa triangle in the horizontal direction (the distance in a plan view)(see FIG. 5)) of about 180 μm (that is equal to or greater than 162 μmand equal to or less than 198 μm), and the surface density (pieces/cm²)of triangular defects with a starting point size (a size in a directionin which the size is the maximum in a plan view) of less than 20 μmamong the triangular defects with a width (the distance from thestarting point to the opposite side of the triangle in the horizontaldirection) of about 180 μm (that is equal to or greater than 162 μm andequal to or less than 198 μm) from the left column when the circulation(gas circulation) is turned on and off. Here, the above-mentionedsurface density of the triangular defects was measured by an opticalmicroscope (MX51 manufactured by Olympus Corporation). The term “alltriangular defects” means all of the triangular defects with anunlimited width (the distance from the starting point to the oppositeside of the triangle in the horizontal direction).

The reason why the triangular defects with a starting point size (a sizein a direction in which the size is the maximum in a plan view) of lessthan 20 μm are separately measured is as follows. It is considered thatthe particles attached onto the SiC substrate include particles floatingin the glove box and particles of a piece of a member which is placed inthe chamber (for example, a ceiling) which falls before epitaxialgrowth. Empirically, the latter particle has a large particle size andthe size of the starting point of the defect caused by the largeparticle size is large. In contrast, the former particle generally has asmaller particle size than the latter particle. When the former particleand the latter particle are classified according to the size of thestarting point, 20 μm is the standard of the classification. Themeasured SiC epitaxial wafer was the first SiC epitaxial wafer after thefurnace was cleaned and the formation conditions of the epitaxial layerwere the same as those in the Example except for the thickness (12.5μm). The SiC substrate was a 4H—SiC single crystal with an off angle of4° and the thickness of the epitaxial layer was 12.5 μm. The width ofthe triangular defect which is formed from the particle attached to theSiC substrate before epitaxial growth as the starting point is 180 μm(=12.5/tan 4°) (see FIG. 5).

TABLE 1 Defect density/cm² All Triangular defects Triangular defectstriangular with a width of with a starting point defects about 180 μmsize of less than 20 μm OFF 1.0 0.33 0.07 ON 1.0 0.27 0

As shown in Table 1, when the circulation was maintained in the onstate, both the surface density of the triangular defects with a widthof about 180 μm and the surface density of the triangular defects with astarting point size of less than 20 μm among the triangular defects witha width of about 180 μm were less than those the circulation was turnedoff. The reason can be considered as follows. The circulation wasmaintained in the on state through the opening and closing of the coverof the SiC-CVD furnace. As a result, it was possible to perform the SiCsubstrate placement operation at low particle density in the glove box.Therefore, the number of particles attached onto the SiC substrate inthe glove box before epitaxial growth was reduced.

In particular, the surface density of the triangular defects with astarting point size of less than 20 μm among the triangular defects witha width of about 180 μm was zero. It is considered that the continuouscirculation through the opening and closing of the cover of the SiC-CVDfurnace is particularly effective in the removal of particles with asmall size in the glove box.

(Triangular Defect Having Piece of Member which is Placed in Chamber asStarting Point)

FIG. 4A shows an optical microscope image of a triangular defect havinga piece of a member which is placed in a typical chamber as a startingpoint. MX51 manufactured by Olympus Corporation was used as the opticalmicroscope.

The SiC epitaxial wafer shown in FIG. 4A was produced by Hot Wall SiCCVD (VP2400HW) manufactured by Aixtron Corporation, which was amulti-wafer susceptor-type (rotation/revolution) SiC-CVD furnace formass production. A shielding plate was not used and a ceiling made ofgraphite was used. The SiC epitaxial wafer in which a SiC epitaxial filmwith a thickness of 10 μm is formed on a 4H—SiC single crystal substratewith an off-angle of 4° is of an eightieth production lot (that is,after a film corresponding to total SiC epitaxial film thickness of 800μm is formed in the chamber).

When the SiC epitaxial wafer is observed by the optical microscope,generally, the optical microscope is focused on the surface of theepitaxial film and a defect on the surface is observed. FIG. 4B shows anoptical microscope image of the same triangular defect when the opticalmicroscope is focused on the surface of the epitaxial film and thedefect on the surface is observed.

In contrast, the inventors shifted the optical microscope focused on thesurface, focused the optical microscope on the interface between the SiCsingle crystal substrate and the epitaxial film, and found a blackforeign material (a black point (“a starting point of a triangulardefect”) shown at the center of a circle) in front (a direction distantfrom the opposite side) of the apex of the triangular defect. Then, theinventors thoroughly analyzed the foreign material, identified theorigin (the piece of the member which is placed in the chamber) of theforeign material, and found a new type of triangular defect having thepiece of the member which is placed in the chamber as the starting pointwhich had not been known before.

FIG. 5 shows a transmission electron microscope (TEM) image obtainedfrom the same SiC epitaxial wafer. HF-2200 manufactured by HitachiHigh-technologies Corporation was used as a transmission electronmicroscope.

The figure shown on the right side of the TEM image in FIG. 5schematically shows the starting point of the triangular defect and thetriangular defect grown from the starting point. A portion which issurrounded by a rectangle indicates a range shown on the TEM image andthe TEM image is an observed image in the vicinity of the starting pointof the triangular defect. The figure shown on the lower side of FIG. 5schematically shows the cross-section in the vicinity of the triangulardefect.

In the case of the triangular defect, the foreign material which is thestarting point of the triangular defect is about 7 μm ahead of the apexof the triangle in the horizontal direction and the distance from thestarting point to the opposite side of the triangle in the horizontaldirection is about 143 μm.

FIG. 6 shows a transmission electron microscope (TEM) image in thevicinity of the starting point (foreign material) of a triangular defectin a SiC epitaxial wafer which is manufactured under the same conditionsas the above-mentioned SiC epitaxial wafer except that a shielding platemade of a graphite base coated with a tantalum carbide (TaC) film isused below the ceiling.

A portion of the triangular defect having the foreign material as thestarting point is made of a 3C—SiC single crystal and a portion which isepitaxially grown normally in the vicinity of the triangular defect ismade of a 4H—SiC single crystal.

FIG. 7A shows the analysis result of the composition of the foreignmaterial, which is the starting point of the triangular defect shown inFIG. 6, by energy dispersive X-ray spectroscopy (EDX).

In FIG. 7A, a peak of 1.711 keV and a peak of 8.150 keV indicatetantalum (Ta). Only the shielding plate is a member made of a materialcontaining tantalum (Ta), which is placed in the chamber. Therefore, itcan be determined that the peaks of tantalum result from tantalum (Ta)of tantalum carbide (TaC) which is the material coating the shieldingplate.

FIG. 7B shows the EDX analysis result of a sample holder by the EDX.

The peaks of Zn, Cu, and the like shown in FIG. 7A result from thematerial of the EDX holder.

As described above, the inventors found a new type of triangular defecthaving, as the starting point, the foreign material (material piece)which was the material forming the member (the shielding plate in FIG.6) which is placed in the chamber and was disposed ahead of the apex ofthe triangular defect (in a direction distant from the opposite side).

When the triangular defects are observed by an optical microscope or anoptical surface inspection apparatus (for example, Candela manufacturedby KLA-Tencor Corporation) using a laser beam, triangular defects with aclear starting point and triangular defects with an unclear startingpoint are observed. In the related art, in many cases, it has beenanalyzed that the triangular defects with the unclear starting point arecaused by the following: since the growth conditions are not satisfied(for example, the growth temperature is too low), it is difficult tonormally perform step-flow growth and a different polytype of crystalnucleus becomes the starting point of the triangular defect. Incontrast, the inventors found that some of the triangular defects withthe unclear starting point were caused by the pieces of the member whichis placed in the chamber. At the present time, it may not be determinedthat all of the triangular defects with the unclear starting point arecaused by the pieces of the member which is placed in the chamber.However, the inventors conceived a technique for reducing the triangulardefects caused by the pieces of the member which is placed in thechamber and succeeded in removing almost all of the triangular defectswith the unclear starting point, which will be described below.Therefore, it is considered that most of the triangular defects with theunclear starting point are caused by the pieces of the member which isplaced in the chamber.

It is possible to determine whether the triangular defect with theunclear starting point is a triangular defect including the pieces ofthe member which is placed in the chamber as the starting point using,for example, a method of shifting the focus of the optical microscopefrom the surface in a depth direction, as described above. Therefore, itis possible to obtain the surface density of the triangular defectshaving the pieces of the member which is placed in the chamber as thestarting point.

Deterioration of the member which is placed in the chamber progresses asfilms are repeatedly formed (the number of production lots increases)and thus the number of falling pieces of the member which is placed inthe chamber increases. Therefore, the surface density of the triangulardefects having the pieces of the member which is placed in the chamberas the starting point increases as the number of times the film isformed increases. Here, the deterioration of the member which is placedin the chamber means the following. An example of the deterioration isthe peeling-off of a film. That is, in the case of the ceiling in whicha graphite base is coated with a tantalum carbide (TaC) film, since thetantalum carbide film and the graphite base have different thermalexpansion coefficients, stress is applied to the tantalum carbide filmdue to a temperature variation caused by the repetition of the formationof a film and the tantalum carbide film peels off. As other examples ofthe deterioration, the tantalum carbide film is cracked and dust of thegraphite base is generated from the crack and the material forming theceiling is sublimated by the mutual interaction between gas in thechamber and the surface of the ceiling. In addition, when SiC isdeposited on the ceiling and a SiC film is grown thereon, the tantalumcarbide film deteriorates due to a difference in the thermal expansioncoefficient between the SiC film and the tantalum carbide film. As such,the surface density of the triangular defects having the member which isplaced in the chamber as the starting point highly depends on the numberof times the film is formed (or the number of production lots). Inaddition, when the number of times the film is formed is greater than apredetermined value (depending on manufacturing conditions), thedeterioration of the member which is placed in the chamber progressesand the surface density of the triangular defects having the memberwhich is placed in the chamber as the starting point increases rapidly.

In contrast, a triangular defect which has damage, such as a polishingflaw in the surface of the substrate (wafer), as a starting point or atriangular defect which has a different polytype of crystal nucleusformed due to unsuitable growth conditions as a starting point does notdepend on the number of times the film is formed. That is, a triangulardefect caused by the substrate or a triangular defect caused by thegrowth conditions does not depend on the number of times the film isformed.

A triangular defect having a downfall as a starting point also dependson the number of times the film is formed. The downfall falls from theceiling when the shielding plate is not used. The downfall falls fromthe shielding plate when the shielding plate is used. When the film isrepeatedly formed, the SiC film formed on the ceiling or the shieldingplate is thick and is likely to peel off. When the shielding plate isused, at least the lower surface of the shielding plate is preferablymade of a material which has higher adhesion to the SiC film than theceiling, which will be described below. Therefore, it is possible toreduce the downfall, as compared to the case in which the shieldingplate is not used.

It is considered that the triangular defects caused by the particles inthe glove box also depend a little on the number of times the film isformed since the contamination of the chamber is spread to the glove boxwhen the substrate is carried in or out of the chamber.

Accordingly, examples of the triangular defects which increase with therepetition of the formation of the film include a triangular defecthaving the downfall as the starting point and a triangular defect causedby a particle in the glove box (as the starting point), in addition tothe triangular defect having the piece of the member which is placed inthe chamber as the starting point. However, the triangular defect havingthe piece of the member which is placed in the chamber as the startingpoint is characterized in that the starting point is unclear in anoptical microscope image or an image (hereinafter, referred to as a“candela image”) obtained by an optical surface inspection apparatususing a laser beam. In contrast, in the triangular defect having thedownfall as the starting point, in many cases, the starting point isclear. Therefore, in general, it is possible to identify the triangulardefect from, for example, the optical microscope image or the candelaimage. Even when triangular defects which are caused by factors otherthan the downfall are included in the triangular defects with theunclear starting point, the surface density of the triangular defectscan be considered to be the upper limit of the surface density of thetriangular defects having the piece of the member which is placed in thechamber as the starting point. Therefore, the surface density of thetriangular defects with the unclear starting point can be managed tomanage the upper limit of the surface density of the triangular defectshaving the piece of the member which is placed in the chamber as thestarting point. An increase in the surface density of the triangulardefects with the unclear starting point due to the repetition of theformation of the film can be managed to manufacture a SiC epitaxialwafer with a low surface density of the triangular defects having thepiece of the member which is placed in the chamber as the startingpoint.

The composition of a foreign material in front of the triangular defectcan be analyzed by, for example, energy dispersive X-ray spectroscopy tostrictly measure and manage the density of the triangular defects havingthe piece of the member which is placed in the chamber as the startingpoint.

At the present time, it is unclear whether the triangular defect causedby the particle in the glove box is included in the triangular defectswith the unclear starting point. When the triangular defect caused bythe particle in the glove box is included in the triangular defects withthe unclear starting point, the percentage of the triangular defectscaused by the particle in the glove box in the triangular defects withthe unclear starting point (whether the triangular defects depend on themanufacturing conditions) is unclear. However, when a substrateplacement process is performed with the particle density reduced in theglove box by the method according to the invention, it is possible toreduce the density of the triangular defects caused by the particles inthe glove box. As a result, it is possible to increase the measurementaccuracy of the surface density of the triangular defects having thepiece of the member which is placed in the chamber as the startingpoint. In addition, it is preferable to apply a method of reducing thesurface density of the triangular defects having the piece of the memberwhich is placed in the chamber as the starting point after particledensity in the glove box is reduced by the method according to theinvention.

When the member which is placed in the chamber deteriorates due to therepetition of the formation of the film, a minute lump peels off fromthe surface of the member which is placed and falls on the wafer or theSiC epitaxial film which is being grown on the wafer. The piece of themember which is placed in the chamber which is the starting point of thetriangular defect is considered as the minute lump. The member which isplaced in the chamber which causes the piece serving as the startingpoint of the triangular defect is mainly a member located above thewafer. It is presumed that the number of pieces which fall on the waferfrom the wall surfaces of the chamber or other members which is placedin the chamber is negligible.

Accordingly, in order to reduce the triangular defects having the pieceof the member which is placed in the chamber as the starting point, theinventors conceived a technique in which members, which are locatedabove a wafer, are coated with a material which generates a small amountof dust or has low sublimation, so as to face the wafer, the surfacedensity of the triangular defects having a piece of a member which isplaced in a chamber as a starting point is measured regularly (forexample, for each production lot or for every plurality of productionlots) or irregularly, and the value of the surface density is managed.In the technique, when the surface density was greater than apredetermined value, the member was replaced and the next SiC epitaxialwafer was manufactured. In this way, it was possible to manufacture aSiC epitaxial wafer with a low surface density of the triangular defectshaving the piece of the member which is placed in the chamber as thestarting point.

The invention provides the following means in order to solve theabove-mentioned problems.

(1) There is provided a method of manufacturing a SiC epitaxial waferincluding a SiC epitaxial layer on a SiC substrate using a SiC-CVDfurnace which is installed in a glove box. The method includes a SiCsubstrate placement step of placing the SiC substrate in the SiC-CVDfurnace while circulating gas in the glove box.

(2) In the method of manufacturing a SiC epitaxial wafer according to(1), the gas may be circulated before and after the SiC substrate isplaced in the SiC-CVD furnace.

(3) In the method of manufacturing a SiC epitaxial wafer according to(2), the gas may be circulated at least 3 minutes before the SiCsubstrate is placed in the SiC-CVD furnace.

(4) In the method of manufacturing a SiC epitaxial wafer according toany one of (1) to (3), the SiC substrate placement step may be performedafter it is checked that particle number density in the glove box isequal to or less than a predetermined value.

(5) In the method of manufacturing a SiC epitaxial wafer according toany one of (1) to (4), the SiC-CVD furnace may include a susceptor thathas a wafer mounting portion on which a wafer is placed, a ceiling thatfaces an upper surface of the susceptor such that a reaction space isformed between the ceiling and the susceptor, and a shielding plate thatis provided so close to a lower surface of the ceiling that a deposit isprevented from being attached to the lower surface of the ceiling. Asurface of the shielding plate which faces the susceptor may be coatedwith a silicon carbide film or a pyrolytic carbon film or the shieldingplate may be made of silicon carbide. The method may further include astep of measuring the surface density of triangular defects including apiece of a member which is placed in the SiC-CVD furnace as a startingpoint in a SiC epitaxial film (layer) of a previously manufactured SiCepitaxial wafer and manufacturing the next SiC epitaxial wafer.

(6) In the method of manufacturing a SiC epitaxial wafer according to(5), when the measurement result shows that the surface density of thetriangular defects including a piece of a member which is placed in theSiC-CVD furnace as the starting point is greater than a predeterminedvalue, the shielding plate may be replaced and the next SiC epitaxialwafer may be manufactured.

(7) In the method of manufacturing a SiC epitaxial wafer according toany one of (1) to (4), the SiC-CVD furnace may include a susceptor thathas a wafer mounting portion on which a wafer is placed and a ceilingthat faces an upper surface of the susceptor such that a reaction spaceis formed between the ceiling and the susceptor. A surface of theceiling which faces the susceptor may be coated with one of a siliconcarbide film and a pyrolytic carbon film, or the ceiling may be made ofsilicon carbide. The method may further include a step of measuring thesurface density of triangular defects including a piece of a memberwhich is placed in the SiC-CVD furnace as a starting point in a SiCepitaxial film (layer) of a previously manufactured SiC epitaxial waferand manufacturing the next SiC epitaxial wafer.

(8) In the method of manufacturing a SiC epitaxial wafer according to(7), when the measurement result shows that the surface density of thetriangular defects including a piece of a member which is placed in theSiC-CVD furnace as the starting point is greater than a predeterminedvalue, the ceiling may be replaced and the next SiC epitaxial wafer maybe manufactured.

(9) There is provided a SiC epitaxial wafer including a SiC epitaxiallayer that is formed on a SiC substrate having an off angle. The surfacedensity of triangular defects, in which a distance from a starting pointto an opposite side in a horizontal direction is equal to or greaterthan (a thickness of the SiC epitaxial layer/tan(x))×90% and equal to orless than (the thickness of the SiC epitaxial layer/tan(x))×110%, in theSiC epitaxial layer is in the range of 0.05 pieces/cm² to 0.5 pieces/cm²(where x indicates the off angle).

(10) In the SiC epitaxial wafer according to (9), the surface density oftriangular defects with a starting point size of less than 20 μm may beequal to or less than 0.05 pieces/cm².

(11) In the SiC epitaxial wafer according to (9) or (10), the surfacedensity of triangular defects including a piece of a member which isplaced in a SiC-CVD furnace as a starting point in the SiC epitaxiallayer may be equal to or less than 0.5 pieces/cm².

(12) In the SiC epitaxial wafer according to claim 10, the piece of themember as the starting point may be made of one of carbon and siliconcarbide.

(13) There is provided a method of manufacturing a SiC epitaxial waferincluding a SiC epitaxial layer on a SiC substrate using a SiC-CVDfurnace which is installed in a glove box. The method includes aparticle number density check step of measuring particle number densityin the glove box and checking whether the particle number density isequal to or less than a predetermined value before the SiC substrate isplaced in the SiC-CVD furnace.

Here, the term “SiC-CVD furnace installed in the glove box” means a typein which a furnace portion for performing epitaxial growth in theSiC-CVD furnace is surrounded by the glove box and the environment ofthe SiC-CVD furnace is connected the internal environment of the glovebox immediately after the cover of the SiC-CVD furnace (chamber) isopened in order to place the SiC substrate and does not include aconstitution in which the glove box and the SiC-CVD furnace areseparated by a gate.

The term “when the SiC substrate is placed in the SiC-CVD furnace” meansthat the cover of the SiC-CVD furnace (chamber) is opened, the inside ofthe SiC-CVD furnace is exposed, and the SiC substrate is placed in theSiC-CVD furnace.

The term “before and after the SiC substrate is placed in the SiC-CVDfurnace” means that the cover of the SiC-CVD furnace (chamber) is openedand closed in order to place the SiC substrate therein.

The term “starting point size” means the size of the starting point in adirection in which the size is the maximum in a plan view.

In the sentence “distance from a starting point to an opposite side in ahorizontal direction is equal to or greater than (a thickness of the SiCepitaxial layer/tan(x))×90% and equal to or less than (the thickness ofthe SiC epitaxial layer/tan(x))×110%”, “90%” and “110%” are setconsidering a variation in the film formation conditions.

Effects of the Invention

According to the invention, it is possible to provide a SiC epitaxialwafer with a low surface density of triangular defects having a particleattached onto a SiC substrate as a starting point and a method ofmanufacturing the same.

According to the invention, it is possible to provide a SiC epitaxialwafer with a low surface density of triangular defects having a piece ofa member which is placed in a chamber as a starting point and a methodof manufacturing the same.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the number ofparticles in a glove box and the surface density of triangular defectswhich is measured in a SiC epitaxial layer of an epitaxial wafer.

FIG. 2 is a graph showing the investigation result of a change in thenumber of particles in the glove box when the circulation (gascirculation) is alternatively and repeatedly turned on and off.

FIG. 3 is a graph showing the relationship between the number ofparticles in the glove box, and the opening and closing of a cover of aSiC-CVD furnace and the turning on and off of circulation.

FIGS. 4A and 4B shows an optical microscope image of a triangular defecthaving a piece of a member which is placed in a typical chamber as astarting point. FIG. 4A shows the optical microscope image when anoptical microscope is focused on a foreign material and FIG. 4B showsthe optical microscope image when the optical microscope is focused onthe surface of an epitaxial film.

FIG. 5 is a plan view and a cross-sectional view schematically showing atransmission electron microscope (TEM) image of the same SiC epitaxialwafer as that shown in FIGS. 4A and 4B and the triangular defect.

FIG. 6 shows a transmission electron microscope (TEM) image of a SiCepitaxial wafer which is manufactured using a shielding plate that ismade of graphite and is coated with a tantalum carbide film.

FIG. 7A shows the measurement result of the same SiC epitaxial wafer asthat shown in FIG. 6 by energy dispersive X-ray spectroscopy and showsthe measurement result of a foreign material.

FIG. 7B shows the measurement result of the same SiC epitaxial wafer asthat shown in FIG. 6 by energy dispersive X-ray spectroscopy and showsthe measurement result of a sample holder.

FIG. 8 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.1 μmand less than 0.15 μm.

FIG. 9 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.15 μmand less than 0.2 μm.

FIG. 10 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.2 μmand less than 0.25 μm.

FIG. 11 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.25 μmand less than 0.3 μm.

FIG. 12 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.3 μmand less than 0.5 μm.

FIG. 13 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.5 μmand less than 1.0 μm.

FIG. 14 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 1.0 μmand less than 5.0 μm.

FIG. 15 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 5.0 μm.

FIG. 16 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.1 μmand less than 0.3 μm.

FIG. 17 is a graph showing the measurement result of the density of thenumber of particles with a size that is equal to or greater than 0.1 μm.

FIG. 18 is a schematic cross-sectional view showing an epitaxial wafermanufacturing apparatus used in the embodiment of the invention.

FIG. 19 is a perspective view showing the lower side of the epitaxialwafer manufacturing apparatus along the line A-A′ of FIG. 18.

FIG. 20 is a schematic enlarged view showing the periphery of ashielding plate shown in FIG. 19.

FIGS. 21A, 21B and 21C show a candela image according to an example.

FIG. 22A shows a candela image according to Comparative Example 1 andFIG. 22B shows a candela image according to Comparative Example 2.

FIG. 23 is a diagram schematically showing the arrangement of a chamber(SiC-CVD furnace) in a glove box.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a SiC epitaxial wafer manufacturing method and a SiCepitaxial wafer to which the invention is applied will be described indetail with reference to the drawings.

In the drawings used in the following description, in some cases,characteristic portions are enlarged in order to facilitate theunderstanding of the characteristics for convenience and, for example,the dimensional ratio of each component is different from the actualdimensional ratio. In addition, in the following description, materialsand dimensions are illustrative examples and the invention is notlimited thereto. The materials and dimensions can be appropriatelychanged without departing from the scope and spirit of the invention.

SiC Epitaxial Wafer Manufacturing Method (First Embodiment)

A SiC epitaxial wafer manufacturing method according to an embodiment ofthe invention is a method which manufactures a SiC epitaxial waferincluding a SiC epitaxial layer on a SiC substrate using a SiC-CVDfurnace that is installed in a glove box and includes a SiC substrateplacement process of placing the SiC substrate in the SiC-CVD furnacewhile circulating gas in the glove box.

It is preferable to perform the circulation of the gas before and afterthe SiC substrate is placed in the SiC-CVD furnace. In addition, it ispreferable to perform the circulation of the gas at least three minutesbefore the SiC substrate is placed in the SiC-CVD furnace.

It is possible to grow a SiC epitaxial film, with particle numberdensity being reduced in the glove box. Therefore, it is possible tomanufacture a SiC epitaxial wafer in which the density (surface density)of triangular defects having existing particles in the glove box, whichare attached to the SiC substrate before the SiC epitaxial film isgrown.

It is preferable that the SiC substrate placement process be performedafter it is checked that the particle number density in the glove box isequal to or less than a predetermined value. In this case, it ispossible to manage the density (surface density) of the triangulardefects having the particles in the glove box, which are attached to theSiC substrate before the SiC epitaxial film is grown, as the startingpoints.

FIGS. 8 to 17 show the measurement results of the number of particlesper cubic feet in the glove box before and after a cover of the SiC-CVDfurnace (chamber) is opened and closed during a SiC substrate placementoperation. Hot Wall SiC CVD (VP2400HW) manufactured by AixtronCorporation was used as the glove box and the SiC-CVD furnace and afilter that had a particle removal ratio of 99.97% or more with respectto particles with a size of 0.3 μm at which the removal ratio was theminimum was used. Aero Track 9110 manufactured by Nitta Corporation wasused as a particle counter. The horizontal axis indicates time (minute)and the vertical axis indicates the particle number density(particles/ft³: the number of particles per cubic feet).

In FIGS. 8 to 17, “♦” (operation 1) indicates the measurement result ofthe particle number density in the glove box in an example of theinvention in which the cover of the SiC-CVD furnace (chamber) is openedand closed to circulate gas 30 minutes before the SiC substrateplacement operation starts (that is, when the cover is opened) and thenthe SiC substrate placement operation is performed. In addition, “□”(operation 2), “Δ” (operation 3), “×” (operation 4), and “*” (operation5) are the measurement results of the particle number density in theglove box in comparative examples in which gas is circulated 30 minutesbefore the SiC substrate placement operation starts and the circulationis stopped when the SiC substrate placement operation starts. In FIGS. 8to 17, the vertical axis between 30 minutes and 31 minutes indicates theboundary between data when the circulation is turned on and data whenthe circulation is turned off in the comparative example.

FIG. 8 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.1 μm and lessthan 0.15 μm. The “size” is based on “size classification” by theparticle counter.

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 10 particles/ft³.

FIG. 9 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.15 μm and lessthan 0.2 μm.

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 10 particles/ft³.

FIG. 10 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.2 μm and lessthan 0.25 μm.

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 6 particles/ft³.

FIG. 11 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.25 μm and lessthan 0.3 μm.

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 10 particles/ft³.

FIG. 12 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.3 μm and lessthan 0.5 μm.

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after 5 minutes elapsedfrom the opening of the cover, the particle number density increasedseveral times although there was a variation in the measurement results.In contrast, in the example (“♦”), there was little change in theparticle number density before and after the SiC substrate placementoperation started and the particle number density was equal to or lessthan 20 particles/ft³.

The difference in the particle number density in this case is less thanthe difference in the particle number density between the example andthe comparative examples when the size is equal to or less than 0.3 μm.

FIG. 13 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.5 μm and lessthan 1.0 μm.

Among the comparative examples (“□”, “Δ”, “×”, and “*”), in somecomparative examples, after the SiC substrate placement operationstarted, that is, after a few minutes elapsed from the opening of thecover, the particle number density increased several times. However, insome comparative examples, there was no change in the particle numberdensity. In contrast, in the example (“♦”), in the case of the size,there was little change in the particle number density before and afterthe SiC substrate placement operation started and the particle numberdensity was equal to or less than 10 particles/ft³.

FIG. 14 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 1.0 μm and lessthan 5.0 μm.

Among the comparative examples (“□”, “Δ”, “×”, and “*”), in somecomparative examples, after the SiC substrate placement operationstarted, that is, after 4 or more minutes elapsed from the opening ofthe cover, the particle number density increased several times. However,in some comparative examples, there was no change in the particle numberdensity. In contrast, in the example (“♦”), in the case of the size,there was little change in the particle number density before and afterthe SiC substrate placement operation started and the particle numberdensity was equal to or less than 10 particles/ft³.

FIG. 15 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 5.0 μm.

In all of the example (“♦”) and the comparative examples (“□”, “Δ”, “×”,and “*”), the particle number density was less than that when the sizewas equal to or less than 5.0 μm and there was no clear differencetherebetween.

FIG. 16 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.1 μm and lessthan 0.3 μm (the sum of the results shown in FIGS. 8 to 11).

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 30 particles/ft³. As such, when the invention is applied,it is possible to reduce the number density of particles with a sizethat is equal to or greater than 0.1 μm and less than 0.3 μm in theglove box to 30 particles/ft³ or less.

FIG. 17 shows the measurement results of the density of the number ofparticles with a size that is equal to or greater than 0.1 μm (the sumof the results shown in FIGS. 8 to 16).

In the comparative examples (“□”, “Δ”, “×”, and “*”), after the SiCsubstrate placement operation started, that is, after a few minuteselapsed from the opening of the cover, the particle number densityincreased several times although there was a variation in themeasurement results. In contrast, in the example (“♦”), there was littlechange in the particle number density before and after the SiC substrateplacement operation started and the particle number density was equal toor less than 60 particles/ft³. As such, when the invention is applied,it is possible to reduce the number density of particles with a sizeequal to or greater than 0.1 μm in the glove box to 60 particles/ft³ orless.

It is considered that the above-mentioned results are affected by theperformance of the filter. A filter which was used in the experimentalresults had a particle removal rate of 99.97% or more with respect toparticles with a size of 0.3 μm at which the removal ratio was theminimum. The invention is effective in the removal of particles with asize of about 0.3 μm. According to the SiC epitaxial wafer manufacturingmethod of the invention, it is possible to manufacture a SiC epitaxialwafer with a low density of triangular defects.

In the SiC epitaxial wafer manufacturing method according to theinvention, when the SiC epitaxial wafers are continuously manufactured,a SiC-CVD furnace is preferably used which includes a susceptor that hasa wafer mounting portion on which a wafer is placed, a ceiling thatfaces the upper surface of the susceptor such that a reaction space isformed between the ceiling and the susceptor, and a shielding plate thatis arranged so close to the lower surface of the ceiling that a depositis prevented from being attached to the lower surface of the ceiling.Preferably, a surface of the shielding plate which faces the susceptoris coated with a silicon carbide film or a pyrolytic carbon film, or theshielding plate is made of silicon carbide. Preferably, the SiCepitaxial wafer manufacturing method includes a process which measuresthe surface density of triangular defects having a material of a memberwhich is placed in the SiC-CVD furnace as a starting point in the SiCepitaxial film of the previously manufactured SiC epitaxial wafer andmanufactures the next SiC epitaxial wafer.

In this case, it is possible to manufacture a SiC epitaxial wafer inwhich the surface density of triangular defects having the materialpiece of the member in the chamber as the starting point is reduced. Inaddition, a structure was used in which the surface of the shieldingplate that faces the susceptor was coated with one of a silicon carbidefilm and a pyrolytic carbon film, or the shielding plate was made ofsilicon carbide. Therefore, the shielding plate is less likely todeteriorate (when the surface of the shielding plate is coated with thesilicon carbide film or when the shielding plate is made of siliconcarbide, the shielding plate is less likely to deteriorate since a SiCfilm deposited on the shielding plate and the film coating the shieldingplate or the shielding plate are made of the same material and there isno difference in their thermal expansion coefficient. When a carbonmaterial base is used as the shielding plate and the shielding plate iscoated with the pyrolytic carbon film, the shielding plate is lesslikely to deteriorate since there is a small difference in the thermalexpansion coefficient between the base and the pyrolytic carbon film)and the number of material pieces which fall from the shielding plateonto the wafer is reduced. In addition, it is possible to use theshielding plate for a long time.

When the measurement result shows that surface density of the triangulardefects having the piece of the member which is placed in the SiC-CVDfurnace as the starting point is greater than a predetermined value, itis preferable to replace the shielding plate and to manufacture the nextSiC epitaxial wafer.

In this case, it is possible to manufacture a SiC epitaxial wafer inwhich the surface density of the triangular defects having the piece ofthe member which is placed in the chamber as the starting point is equalto or less than a predetermined value.

In the SiC epitaxial wafer manufacturing method according to theinvention, when the SiC epitaxial wafer is continuously manufactured, aSiC-CVD furnace is preferably used which includes a susceptor that has awafer mounting portion on which a wafer is placed and a ceiling thatfaces the upper surface of the susceptor such that a reaction space isformed between the ceiling and the susceptor. Preferably, a surface ofthe ceiling which faces the susceptor is coated with one of a siliconcarbide film and a pyrolytic carbon film, or the ceiling is made ofsilicon carbide. Preferably, the SiC epitaxial wafer manufacturingmethod includes a process which measures the surface density oftriangular defects having a material piece of a member in the SiC-CVDfurnace as a starting point in the SiC epitaxial film of the previouslymanufactured SiC epitaxial wafer and manufactures the next SiC epitaxialwafer.

In this case, it is possible to manufacture a SiC epitaxial wafer inwhich the surface density of triangular defects having a material pieceof a member in the chamber as a starting point. In addition, since thestructure is used in which the surface of the ceiling that faces thesusceptor is coated with one of a silicon carbide film and a pyrolyticcarbon film, or the ceiling is made of silicon carbide, the ceiling isless likely to deteriorate (when the ceiling is coated with the siliconcarbide film or when the ceiling is made of silicon carbide, the ceilingis less likely to deteriorate since a SiC film deposited on the ceilingand the film coating the ceiling or the ceiling are made of the samematerial and there is no difference in thermal expansion coefficient.When a carbon material base is used as the ceiling and the ceiling iscoated with the pyrolytic carbon film, the ceiling is less likely todeteriorate since there is a small difference in the thermal expansioncoefficient between the base and the pyrolytic carbon film) and thenumber of material pieces which fall from the ceiling onto the wafer isreduced. In addition, it is possible to use the ceiling for a long time.

When the measurement result shows that the surface density of thetriangular defects having the piece of the member which is placed in theSiC-CVD furnace as the starting point is greater than a predeterminedvalue, it is preferable to replace the ceiling and to manufacture thenext SiC epitaxial wafer.

In this case, it is possible to manufacture a SiC epitaxial wafer inwhich the surface density of the triangular defects having the piece ofthe member which is placed in the chamber as the starting point is equalto or less than a predetermined value.

<SiC Substrate>

It is preferable that the SiC substrate be made of a SiC single crystal.Any polytype can be used as the SiC single crystal substrate. 4H-SiCwhich is mainly used to manufacture a practical SIC device can be used.A SiC single crystal substrate which is processed from a bulk crystalproduced by, for example, a sublimation method is used as a substrate ofthe SiC device. In general, a SiC epitaxial layer which serves as anactive region of the SiC device is formed on the SiC single crystalsubstrate by a chemical vapor deposition (CVD) method.

Any off-angle may be used as the off-angle of the SiC single crystalsubstrate. The off-angle is not particularly limited. However, a smalloff-angle of, for example, 0.4° to 5° is preferable in order to reducecosts. The angle of 0.4° is the lower limit of the off-angle at whichstep-flow growth can be achieved.

When the SiC single crystal substrate has a size of up to about 2inches, the off-angle of the SiC single crystal substrate is mainly 8°.At this off-angle, the terrace width of the surface of the wafer issmall and the step-flow growth can be easily achieved. However, thenumber of wafers obtained from a SiC ingot decreases as the off-angleincreases. Therefore, the off-angle of a SiC substrate with a size of 3inches or more is mainly about 4°.

As the off-angle decreases, the terrace width of the surface of the SiCsingle crystal substrate increases. Therefore, a variation is likely tooccur in the speed at which migration atoms are taken at a step end,that is, the growth speed of the step end. As a result, a step with ahigh growth speed overtakes a step with a low growth speed and iscombined with the step with a low growth speed and step bunching islikely to occur. For example, in a substrate with an off-angle of 0.4°,the terrace width is 10 times more than the terrace width of a substratewith an off-angle of 4° and the length of the step-flow growth isincreased by one digit. Therefore, it is noted that step-flow growthconditions used in the substrate with an off-angle of 4° needs to beadjusted.

A SiC single crystal substrate in which a growth surface of a SiCepitaxial layer is processed in a convex shape can be used.

When the SiC epitaxial wafer is manufactured (the SiC epitaxial layer isformed (grown)), the rear surface of the SiC single crystal substrate isdirectly heated from a heated susceptor. However, the front surface (thesurface on which the SiC epitaxial layer is formed) is exposed to avacuum space and is not directly heated. Since hydrogen which is acarrier gas flows on the front surface, heat is dissipated. From thissituation, the temperature of the front surface is lower than that ofthe rear surface during epitaxial growth. The magnitude of thermalexpansion of the front surface is less than that of the rear surface dueto the difference in temperature and the front surface of the SiC singlecrystal substrate is deformed in a concave shape during epitaxialgrowth. Therefore, when the SiC single crystal substrate in which thegrowth surface of the SiC epitaxial layer is processed in a convex shapeis used, epitaxial growth can be performed in a state in which theconcaveness (warping) of the SiC single crystal substrate duringepitaxial growth is removed.

The thickness of the SiC epitaxial layer is not particularly limited.However, for example, when the SiC epitaxial layer is formed for 2.5hours at a typical growth speed of 4 μm/h, the thickness of the SiCepitaxial layer is 10 μm.

<SiC-CVD Furnace>

The SiC-CVD furnace is not particularly limited.

FIG. 18 is a schematic cross-sectional view showing a portion of theSiC-CVD furnace which is used in the SiC epitaxial wafer manufacturingmethod according to the invention. FIG. 19 is a perspective view showingthe lower side of the SiC-CVD furnace along the line A-A′ of FIG. 18.FIG. 20 is an enlarged view schematically showing the periphery of theshielding plate shown in FIG. 18.

The SiC-CVD furnace which is used in the SiC epitaxial wafermanufacturing method according to the invention is, for example, aSiC-CVD furnace 100 shown in FIG. 18 that forms an epitaxial layer onthe surface of a wafer while being supplied with a raw material gas andincludes a plurality of mounting portions 2 b on which wafers areplaced, a susceptor 2 on which the plurality of mounting portions 2 bare arranged in a line in a circumferential direction, a ceiling 3 whichfaces the upper surface of the susceptor 2 such that a reaction space 4is formed between the ceiling 3 and the susceptor 2, and a shieldingplate 10 that is arranged so close to the lower surface of the ceiling 3that a deposit in a gas phase is prevented from being attached to thelower surface of the ceiling 3. A surface of the shielding plate 10which faces the susceptor 2 is coated with one of a silicon carbide filmand a pyrolytic carbon film, or the shielding plate 10 is made ofsilicon carbide.

For example, a raw material gas which includes, for example, silane(SiH₄) as a Si source and propane (C₃H₈) as a C source can be used. Inaddition, a carrier gas including hydrogen (H₂) can be used.

The SiC-CVD furnace 100 according to this embodiment further includesheating means 6 and 7 which are located below the susceptor 2 and abovethe ceiling, respectively, and heat the wafers placed on the mountingportions 2 b, and a gas introduction pipe 5 which has a gas inletthrough which the raw material gas is introduced from a central portionof the upper surface of the ceiling 3 to the reaction space 4 andsupplies the raw material gas discharged from the gas inlet from theinside to the outside of the reaction space 4.

The heating means 6 and 7 are induction coils, can heat the ceiling 3using high-frequency induction heating of the induction coils, can heatthe shielding plate 10 using radiation heat from the heated ceiling 3,and can heat the wafers using radiation heat from the shielding plate10.

In this embodiment, the wafer is heated by the heating means which arelocated below the susceptor 2 and above the ceiling. However, theheating means may be arranged only on the lower surface side of thesusceptor 2.

Heating means for the SiC single crystal substrate is not limited to theabove-mentioned heating means using high-frequency induction heating,but heating means using resistance heating may be used.

The ceiling 3 is supported by a supporting member 13 which is fixed tothe gas introduction pipe 5 through a protrusion portion 12 thatprotrudes from a central portion of the lower surface of the ceiling 3so as to be disposed in an opening portion 10 b of the shielding plate10. The protrusion portion 12 makes it difficult for gas to flow from aninner circumferential portion of the shielding plate 12 to a spacebetween the shielding plate 10 and the ceiling 3.

As the ceiling 3, the following can be used: a ceiling which is made ofa carbon material such as graphite; a ceiling which is made of siliconcarbide; or a ceiling formed by coating a base made of a carbon materialwith a film made of SiC, pyrolytic carbon, TaC, or the like. The ceiling3 is preferably made of a material which is less likely to be sublimateddue to dust generation at a high temperature or mutual interaction withgas in the chamber.

The shielding plate 10 is configured so as to be detachably attached inthe chamber. In this embodiment, an outer circumference portion 10 a ofthe shielding plate 10 is placed on a supporting portion 11 that isprovided on the surface of the inner wall of the chamber.

Since only the outer circumferential portion of the shielding plate 10is supported, it is possible to detachably attach the shielding plate 10in the chamber while avoiding the contact between the gas introductionpipe 5 which is maintained at a low temperature in order to introducethe raw material gas without being decomposed and the innercircumferential portion (the central portion in which the openingportion is formed) of the shielding plate 10 which is heated to a hightemperature by the heating means.

The shielding plate 10 is preferably divided into a plurality of parts.In this embodiment, the shielding plate 10 is divided into two parts,that is, a pair of members 10A and 10B along the central line, as shownin FIG. 19. In this case, each of the pair of members 10A and 10B can beplaced on the supporting portion 11. At the time of replacement, eachmember may be detached from the supporting portion 11. Therefore,operability is improved and the risk of damage is reduced duringplacement, replacement, and maintenance.

In addition, when the shielding plate 10 is divided into a plurality ofparts, heat stress is reduced and warping or deformation is suppressed.

The shielding plate 10 generally prevents dust (graphite) from fallingfrom the lower surface of the ceiling 3 which is made of graphite to thewafer to reduce the surface density of triangular defects having amaterial piece of the ceiling as a starting point. However, when amaterial piece of the shielding plate 10 falls to the wafer, thetriangular defect having the material piece of the shielding plate asthe starting point occurs. In order to suppress the occurrence of thetriangular defect, the shielding plate 10 needs to be made of a materialwhich is less likely to be sublimated due to dust emission at a hightemperature or mutual interaction with gas in the chamber than thematerial forming the ceiling 3. Therefore, a shielding plate which ismade of silicon carbide or a shielding plate formed by coating agraphite base with one of a silicon carbide film and a pyrolytic carbonfilm is used as the shielding plate 10.

The thickness of the silicon carbide film or the pyrolytic carbon filmis preferably equal to or greater than 20 μm in order to suppressdeterioration. In addition, the film thickness is preferably equal to orless than 100 μm in order to reduce stress based on a difference inthermal expansion coefficient from the graphite base.

At least the lower surface of the shielding plate 10 is preferably of amaterial with high adhesion to the SiC film in order to reduce theamount of downfall of the SiC film which is deposited in a gas phase,peels off, and falls to the SiC single crystal substrate or the SiCepitaxial layer. An example of this material is silicon carbide. Anexample of the shielding plate is a shielding plate having a surfacecoated with a silicon carbide film.

In this embodiment, the shielding plate 10 is preferably made of amaterial with high thermal conductivity since it needs to receive heatradiation from the ceiling 3, to be heated, to emit radiation heat, andto heat the wafer.

When the shielding plate 10 is made of silicon carbide, it can bemanufactured by, for example, a chemical vapor deposition (CVD) methodor sintering. However, the CVD method is preferable for manufacturing ashielding plate with a high-purity material.

It is preferable to roughen the surface of the shielding plate 10 using,for example, polishing in order to increase the adhesion of the surface,on which silicon carbide is deposited, to silicon carbide.

The thickness of the shielding plate 10 is preferably in the range of 2mm to 6 mm in order to prevent a crack. The reason is as follows. Theshielding plate 10 is easily cracked. When the thickness of theshielding plate 10 is less than 2 mm, the shielding plate 10 isexcessively bent and cracked. When the thickness is greater than 6 mm,the shielding plate 10 is also cracked.

When the ceiling is made of silicon carbide, the thickness of the memberis less than that of the ceiling and the shielding plate can beresistant to thermal strain even though the same material is used.Therefore, the shielding plate is less likely to be cracked.

The plurality of mounting portions 2 b are arranged in a line in thecircumferential direction on the disk-shaped susceptor 2 so as tosurround the central portion of the susceptor 2. A rotating shaft 2 afor revolution is attached to a central portion of the lower surface ofthe susceptor 2. The rotating shaft 2 a for revolution is arrangedimmediately below the gas introduction pipe 5. A rotating shaft (notshown) for rotation is attached to each mounting portion 2 b.

In this way, the susceptor 2 resolves the SiC single crystal substrateabout the gas introduction pipe 5 and the SiC single crystal substrateand the mounting portion 2 b are rotated about the center of the SiCsingle crystal substrate.

In the SiC-CVD furnace, since a cooling gas is introduced from the gasintroduction pipe 5 which is provided at the central portion or it isdifficult to perform induction heating in the central portion of thesusceptor 2, the temperature of the susceptor 2 is generally reducedtoward to the central portion. Then, the temperature of the outercircumference of the rotating mounting portion 2 b, that is, the outercircumference of the SiC single crystal substrate provided on themounting portion 2 b is reduced by the influence of the reduction in thetemperature of the susceptor. Therefore, in a general susceptor-typeSiC-CVD furnace, the provided SiC single crystal substrate (wafer) has atemperature gradient in which the temperature of a central portion ofthe wafer is the highest and is reduced toward the outer circumferentialportion of the wafer. The temperature gradient of the SiC single crystalsubstrate causes compressible stress in the central portion of the SiCsingle crystal substrate during an epitaxial growth process.

A flange portion 9 a which protrudes in a direction in which a diameterincreases is provided at the leading end (lower end) of the gasintroduction pipe 5. The flange portion 5 a is used to make a rawmaterial gas G, which is vertically discharged downward from the lowerend of the gas introduction pipe 9, radially flow in the horizontaldirection in a space between the flange portion 5 a and the susceptor 2facing the flange portion 5 a.

In the CVD apparatus 100, the raw material gas G which is dischargedfrom the lower end of the gas introduction pipe 9 flows radially fromthe inside to the outside of the reaction space 4. Therefore, it ispossible to supply the raw material gas G in parallel to the in-plane ofthe SiC single crystal substrate. In addition, unnecessary gas in thechamber can be discharged from an exhaust port (not shown) which isprovided in the wall of the chamber to the outside of the chamber.

Here, the ceiling 3 is heated to a high temperature by the inductioncoils 7, but the inner circumferential portion (the central portion inwhich the opening portion 10 b is formed) does not come in contact withthe gas introduction pipe 9 which is at a low temperature in order tointroduce the raw material gas G. In addition, the inner circumferentialportion of the ceiling 3 is placed on the supporting member 11 which isattached to the outer circumferential portion of the gas introductionpipe 9 and the ceiling 3 is supported upward in the vertical direction.The ceiling 3 can be moved in the vertical direction.

The temperature gradient of the SiC single crystal substrate variesdepending on the flow rate of the gas to be introduced or the positionof the induction heating coils. In this embodiment, the flow rate of thegas to be introduced or the position of the induction heating coils arepreferably adjusted such that the SiC single crystal substrate has atemperature gradient in which the temperature is the lowest in thecentral portion of the wafer and increases toward the outercircumferential portion of the wafer.

A distance d1 between the upper surface of the shielding plate 10 andthe lower surface of the ceiling 3 is preferably set in the range of 0.5mm to 1 mm. In this case, the shielding plate 10 prevents a SiC depositfrom being deposited on the lower surface of the ceiling 3.

The shielding plate 10 is arranged so as to be separated from the innerwall 1 a of the chamber 1 and a distance d2 between an outercircumferential surface 10 c of the shielding plate 10 and the innerwall 1 a of the chamber 1 in the horizontal direction is preferably inthe range of 1.0 mm to 3.0 mm. In this case, the shielding plate 10 isprevented from coming into contact with the wall surface 1 a due tothermal expansion during heating.

For example, when the shielding plate is made of silicon carbide, thedistance is preferably in a range of 1.0 mm to 2.0 mm.

A distance d3 from an inner wall 10 d of the opening portion 10 b of theshielding plate 10 to the outer wall 5 a of the gas introduction pipe 5is preferably set in the range of 0.5 mm to 1 mm. In this case, theshielding plate 10 is prevented from coming into contact with theprotrusion portion 12 due to thermal expansion during heating and gas isless likely to flow from the inner circumferential side of the shieldingplate 10 to the space between the shielding plate 10 and the ceiling 3

A distance d4 between the inner circumferential surface of the ceiling 3and the outer circumferential surface of the gas introduction pipe 9 ispreferably set in the range of 0.5 mm or less. In this case, the gasintroduction pipe 5 which is at a low temperature in order to introducethe raw material gas G is affected by radiation heat from the ceiling 3which is heated at a high temperature by the induction coils 7 as littleas possible.

In this embodiment, as shown in FIG. 20, the ceiling 3 includes theprotrusion portion 12 which protrudes from the inner circumferentialportion 3 d of the lower surface 3 c between the inner wall 10 d of theopening portion 10 b of the shielding plate 10 and the outer wall 5 a ofthe gas introduction pipe 5. The protrusion portion 12 may be formedintegrally with the ceiling 3 or separately from the ceiling 3. Theprotrusion portion 12 is preferably arranged along the inner wall 10 din the opening portion 10 b of the shielding plate 10. FIGS. 5 and 7show only the cross-section of a part of the protrusion portion.

The protrusion portion 12 can prevent the gas of the film material in agas phase from flowing from the gap between the inner wall 10 d of theopening portion 10 b of the shielding plate 10 and the outer wall 5 a ofthe gas introduction pipe 5 and from being deposited on the ceiling.

In this embodiment, the protrusion portion 12 is provided. However, theprotrusion portion 12 may not be provided.

The SiC-CVD furnace according to this embodiment includes the shieldingplate 10. Therefore, even when a material piece of the ceiling 3 falls,the shielding plate 10 can receive the material piece and prevent itfrom falling on the wafer or the epitaxial layer. Therefore, it ispossible to reduce the surface density of triangular defects having thepiece of the ceiling 3 as a starting point in the epitaxial layer. Inaddition, since at least the lower surface of the shielding plate 10 iscoated with one of a silicon carbide film and a pyrolytic carbon film,or the shielding plate 10 is made of silicon carbide, the number ofmaterial pieces falling from the shielding plate 10 is less than thatfalling from the ceiling.

Therefore, when the SiC-CVD furnace is used, it is possible tomanufacture a SiC epitaxial wafer with a low surface density oftriangular defects having a piece of a member which is placed in thechamber as a starting point, as compared to a case in which the SiC-CVDfurnace according to the related art is used.

A SiC-CVD furnace without the shielding plate may be used.

SiC Epitaxial Wafer Manufacturing Method (Second Embodiment)

A SiC epitaxial wafer manufacturing method according to a secondembodiment manufactures a SiC epitaxial wafer including a SiC epitaxiallayer on a SiC substrate using a SiC-CVD furnace which is installed in aglove box and includes a particle number density check process whichmeasures particle number density in the glove box and checks whether theparticle number density is reduced to a predetermined value or lessbefore the SiC substrate is placed in the SiC-CVD furnace.

In the SiC epitaxial wafer manufacturing method, it is possible tomanage the density (surface density) of triangular defects havingparticles, which are attached to the SiC substrate in the glove boxbefore the SiC epitaxial film is grown, as starting points. Therefore,it is possible to manufacture a SiC epitaxial wafer in which the density(surface density) of the triangular defects is reduced.

<SiC Epitaxial Wafer>

According to the SiC epitaxial wafer manufacturing method of anembodiment of the invention, it is possible to obtain a SiC epitaxialwafer which includes a SiC epitaxial layer on a SIC single crystalsubstrate having an off-angle and in which the surface density oftriangular defects having particles, which are attached to the SiCsubstrate, as starting points in the SiC epitaxial layer is in the rangeof 0.1 to 2.0 pieces/cm².

A SiC epitaxial wafer according to another embodiment of the inventionincludes a SiC epitaxial layer on a SiC substrate having an off angle.In the SiC epitaxial layer, the surface density of triangular defects inwhich a distance from a starting point to an opposite side in thehorizontal direction is equal to or greater than (the thickness of theSiC epitaxial layer/tan(x))×90% and equal to or less than (the thicknessof the SiC epitaxial layer/tan(x))×110% is 0.05 pieces/cm² to 0.5pieces/cm² (where x indicates the off angle). As shown in Table 1, it isalso possible to manufacture a SiC epitaxial wafer in which the surfacedensity of the triangular defects is 0.27 pieces/cm². From this result,it is also possible to manufacture a SiC epitaxial wafer in which thesurface density of the triangular defects is equal to or greater than0.25 pieces/cm² and equal to or less than 0.5 pieces/cm².

In the SiC epitaxial wafer according to the invention, the surfacedensity of triangular defects with a starting point size less than 20 μmis preferably equal to or less than 0.05 pieces/cm². As shown in Table1, it is also possible to manufacture a SiC epitaxial wafer in which thesurface density of triangular defects with a starting point size lessthan 20 μm is 0 pieces/cm². From this result, it is also possible tomanufacture a SiC epitaxial wafer in which the surface density oftriangular defects is equal to or greater than 0 pieces/cm² to equal toor less than 0.05 pieces/cm².

In the SiC epitaxial wafer according to the invention, in the SiCepitaxial layer, the surface density of triangular defects having apiece of a member which is placed in the SiC-CVD furnace as a startingpoint is preferably equal to or less than 0.5 pieces/cm². In addition,the piece as the starting point may be made of carbon or siliconcarbide.

Hereinafter, in the SiC epitaxial wafer manufacturing method accordingto the first embodiment of the invention, an example of a series ofprocesses including a process of placing a SiC substrate, a process offorming an epitaxial layer, and a process of measuring the surfacedensity of triangular defects and replacing members when the SiCepitaxial wafers are continuously manufactured will be described.

The example of the processes can also be appropriately applied to a SiCepitaxial wafer manufacturing method according to another embodiment ofthe invention.

First, a polishing process is performed as pre-processing for the SiCsubstrate.

<Polishing Process>

In the polishing process, the 4H-SiC single crystal substrate (SiCsubstrate) which remains on the surface of the wafer in a slice processis polished until the thickness of a lattice disorder layer on thesurface of the wafer is 3 nm or less.

The “lattice disorder layer” means a layer in which a striped structurecorresponding to an atomic layer (lattice) of the SiC single crystalsubstrate or a portion of a stripe in the striped structure is not clearin a lattice image (an image in which a crystal lattice can beconfirmed) of a TEM (see PTL 5).

The polishing process includes a plurality of polishing processes, suchas rough polishing which is generally called lapping, precise polishingwhich is called polishing, and chemical mechanical polishing(hereinafter, referred to as CMP) which is called ultraprecisepolishing. In mechanical polishing before the CMP, preferably,processing pressure is equal to or less than 350 g/cm² and abrasiveparticles with a diameter of 5 μm or less are used to suppress thethickness of a damage layer (not only a damage which can be detected asthe “lattice disorder layer” in the TEM, but also a portion in which,for example, the distortion of the lattice that cannot be detected bythe TEM is deeply present) to 50 nm. Furthermore, in the CMP, polishingslurry preferably includes abrasive particles with an average particlediameter of 10 nm to 150 nm and an inorganic acid. In this case, thepolishing slurry preferably has a pH of 2 or less at 20° C. The abrasiveparticle is silica and 1 mass % to 30 mass % of silica is preferablycontained. The inorganic acid is more preferably at least one of ahydrochloric acid, a nitric acid, a phosphoric acid, and a sulfuricacid.

<SiC Substrate Placement Process>

Next, in a SiC substrate placement process, the cover of the furnace isopened and a SiC substrate (wafer) is placed on the wafer mountingportion while gas in the glove box is being circulated. After the SiCsubstrate is placed, the cover of the furnace is closed. It ispreferable that the gas be continuously circulated by the opening andclosing of the cover.

It is preferable to use inert gas, such as argon, as the gas in theglove box.

The gas can be circulated by a circulation device through a filterprovided in the glove box.

After the cover of the furnace is closed and before epitaxial growth isperformed, the circulation may be turned off.

<Particle Number Density Check Process>

Before the SiC substrate is placed in the SiC-CVD furnace, a particlenumber density check process may be performed which measures particlenumber density in the glove box and checks whether the particle numberdensity is equal to or less than a predetermined value.

<Cleaning (Gas Etching) Process>

In a cleaning process, the substrate subjected to the polishing andconvex processing is heated at 1400 C.° to 1800° in a hydrogenatmosphere and cleaning (gas etching) is performed on the surface of thesubstrate.

The gas etching is performed for 5 minutes to 30 minutes under theconditions that the SiC single crystal substrate is maintained at 1400C.° to 1800° C., the flow rate of hydrogen gas is in the range of 40 slmto 120 slm, and pressure is in the range of 100 mbar to 250 mbar.

After the polished SiC single crystal substrate is cleaned, it is set inan epitaxial growth apparatus, for example, a multiple-sheet planetaryCVD apparatus for mass production. After hydrogen gas is introduced intothe apparatus, pressure is adjusted to 100 mbar to 250 mbar. Then, thetemperature of the apparatus increases so that the temperature of thesubstrate is in the range of 1400 C.° to 1600° C., preferably, equal toor greater than 1480° C. Then, gas etching is performed on the surfaceof the substrate with the hydrogen gas for 1 minute to 30 minutes. Whenthe gas etching is performed with the hydrogen gas under theseconditions, the amount of etching is about 0.05 μm to 0.4 μm.

SiH4 gas and/or C3H8 gas may be added to the hydrogen gas. In somecases, short step bunching occurs incidentally in a shallow pit which iscaused by spiral dislocation. SiH₄ gas with a concentration of less than0.009 mol % is added to the hydrogen gas in order to form a Si-richenvironment in a reactor and the gas etching is performed such that thedepth of the shallow pit is small. Therefore, it is possible to suppressthe incidental occurrence of the short step bunching in the shallow pit.

When SiH₄ gas and/or C₃H₈ gas is added, it is preferable that thechamber be evacuated to change the atmosphere to a hydrogen gasatmosphere before a film is formed (epitaxial growth).

<Film Forming (Epitaxial Growth) Process>

In a film forming (epitaxial growth) process, the amount ofcarbon-containing gas and the amount of silicon-containing gas which arerequired to epitaxially grow silicon carbide are supplied at apredetermined concentration ratio (for example, the concentration ratioC/Si of SiH₄ gas and/or C₃H₈ gas is 0.7 to 1.2) to the surface of thecleaned substrate (after the temperature increases when the growthtemperature of the epitaxial film is higher than the cleaning (gasetching) temperature) to epitaxially grow a SiC film.

It is preferable to simultaneously supply the carbon-containing gas andthe silicon-containing gas. In this case, the step bunching isconsiderably reduced.

Here, the “simultaneous supply” means that the gases are not necessarilysupplied at exactly the same time, but a difference between the supplytimes of the gases is within several seconds.

The flow rate of the SiH₄ gas is in the range of 15 sccm to 150 sccm,the flow rate of the C₃H₈ gas is in the range of 3.5 sccm to 60 sccm,pressure is in the range of 80 mbar to 250 mbar, the growth temperatureis greater than 1600° C. and equal to or less than 1800° C., and thegrowth speed is in the range of 1 μm to 20 μm per hour. These conditionsare determined by controlling the off-angle, the film thickness, theuniformity of carrier concentration, and the growth speed. Nitrogen gasis introduced as a doping gas at the same time as the formation of afilm starts to control carrier concentration in the epitaxial layer.

As a method of suppressing the step bunching during the formation of afilm, a method has been known which reduces the concentration ratio C/Siof the raw material gas to be supplied in order to increase themigration of Si atoms on the surface of the film. In the invention, theconcentration ratio C/Si is in the range of 0.7 to 1.2. In general, theepitaxial layer to be grown has a thickness of about 5 μm to about 20 μmand a carrier concentration of about 2×10¹⁵ cm⁻³ to 15×10¹⁵ cm³.

The growth temperature is in the range of 1400° C. to 1800° C. However,preferably, the growth temperature is equal to or greater than 1600° C.in order to reduce a stacking fault. In addition, preferably, the growthspeed increases as the growth temperature increase. Preferably, thegrowth speed increases as the off-angle of the SiC single crystalsubstrate increases at the same growth temperature.

For example, (1) when a 4H-SiC single crystal substrate with anoff-angle of 0.4° to 2° is used, the following relationship isestablished between the growth temperature for epitaxially growing thesilicon carbide film and the growth speed: when the growth temperatureis in the range of 1600° C. to 1640° C., the growth speed is in therange of 1 μm/h to 3 μm/h; when the growth temperature is in a range of1640° C. to 1700° C., the growth speed is in a range of 3 μm/h to 4μm/h; and when the growth temperature is in a range of 1700° C. to 1800°C., the growth speed is in a range of 4 μm/h to 10 μm/h.

(2) When a 4H—SiC single crystal substrate with an off-angle of 2° to 5°is used, the following relationship is established between the growthtemperature for epitaxially growing the silicon carbide film and thegrowth speed: when the growth temperature is in a range of 1600° C. to1640° C., the growth speed is in a range of 2 μm/h to 4 μm/h; when thegrowth temperature is in a range of 1640° C. to 1700° C., the growthspeed is in a range of 4 μm/h to 10 μm/h; and when the growthtemperature is in a range of 1700° C. to 1800° C., the growth speed ispreferably in a range of 10 μm/h to 20 μm/h.

<Temperature Reduction Process>

In a temperature reduction process, it is preferable to stop the supplyof the carbon-containing gas and the silicon-containing gas (forexample, a SiH₄ gas and a C₃H₈ gas) at the same time in order tosuppress the deterioration of morphology. After the supply of the gas isstopped, the temperature of the substrate is maintained until thecarbon-containing gas and the silicon-containing gas are exhausted.Then, the temperature is reduced.

<Processes of Measuring Surface Density of Triangular Defect andReplacing Member>

After the surface density of triangular defects having a piece of amember which is placed in the chamber as a starting point in a SiCepitaxial layer of a SiC epitaxial wafer is measured regularly (forexample, for each production lot or every plurality of production lots)or irregularly, the next SiC epitaxial wafer is manufactured.

The surface density of the triangular defects having the piece of themember which is placed in the chamber as the starting point can beobtained, for example, by shifting the position of a focus from thesurface of the epitaxial layer of the SiC epitaxial wafer to theinterface (in the depth direction of the film) between the epitaxiallayer (film) and the SiC single crystal substrate, finding thetriangular defect having the piece of the member which is placed in thechamber as the starting point, and measuring the triangular defect,using an optical microscope. In this embodiment, it is considered thatthe kind (type) of triangular defects which increase with the repetitionof the film forming process includes only a triangular defect having thepiece of the member which is placed in the chamber as the starting pointand a triangular defect having downfall as the starting point. Ingeneral, the starting point of the triangular defect having the downfallas the starting point is clear, but the starting point of the triangulardefect having the piece of the member which is placed in the chamber asthe starting point is unclear. It is possible to manufacture a SiCepitaxial wafer with a low surface density of the triangular defectshaving the piece of the member which is placed in the chamber as thestarting point by identifying the triangular defect having the downfallas the starting point and managing an increase in the surface density onthe basis of the characteristics. When the density of the triangulardefects having the piece of the member which is placed in the chamber asthe starting point is desired to be strictly measured and managed, thecomposition of a foreign material in the front of the triangular defectcan be analyzed by, for example, energy dispersive X-ray spectroscopy.

When the measurement result shows that the surface density of thetriangular defects having the piece of the member which is placed in thechamber as the starting point is greater than a predetermined value, itis preferable to replace the shielding plate 10 and manufacture the nextSiC epitaxial wafer. For example, when the surface density of thetriangular defects in the previously manufactured SiC epitaxial wafer isabout 0.25 pieces/cm², the shielding plate 10 is replaced and the nextSiC epitaxial wafer is manufactured. Then, it is possible to manufacturea SiC epitaxial wafer in which the surface density of the triangulardefects is reliably equal to or less than 0.5 pieces/cm².

EXAMPLES

Hereinafter, the effect of the invention will be described in detailusing examples. The invention is not limited to these examples.

Example

In the SiC-CVD furnace shown in FIG. 18, the ceiling which was made ofgraphite was used and the shielding plate (a diameter of 371 mm and athickness of 4 mm) which was divided into two parts shown in FIG. 19 andin which a graphite base was coated with a silicon carbide film wasused. The shielding plate was arranged at a distance (d1) of 0.5 mm fromthe ceiling.

A SiC single crystal substrate which had, as a main surface, a Sisurface in which a c-plane ((0001) plane) was inclined at an angle of 4°in the <11-20> direction, a diameter of 3 inches (76.2 mm), and athickness of 350 μm was used as the 4H-SiC single crystal substrate.

Then, the polishing process was performed on the SiC single crystalsubstrate and organic solvent cleaning, acid and alkali cleaning, andsufficient water washing were performed as preprocessing on the SiCsingle crystal substrate.

The cover of the SiC-CVD furnace was opened, with circulation turned onin the glove box, and the SiC single crystal substrate was placed on thewafer mounting portion. Then, the cover was closed and the SiC-CVDfurnace was evacuated. Then, hydrogen gas was introduced to adjust theatmosphere of the SiC-CVD furnace to a reduced pressure atmosphere of200 mbar. Then, the temperature increased to 1570° C. and growth wasperformed at a growth speed of 5 μm/h for one hour to form a SiCepitaxial film with a thickness of 5 μm. In this way, a SiC epitaxialwafer was manufactured.

Hydrogen was used as the carrier gas, a mixed gas of SiH₄ and C₃H₈ wasused as the raw material gas, and N₂ was supplied as a dopant.

The SiC epitaxial wafer was repeatedly manufactured under the aboveconditions without replacing the member which is placed in the chamber.FIGS. 21A and 21B show the candela image of a second production lot andthe candela image of an eightieth production lot. Black spots indicatetriangular defects.

The number of all kinds of triangular defects was measured from thecandela images and the surface density of all kinds of triangulardefects was obtained.

In addition, the number of triangular defects having the piece of themember which is placed in the chamber as the starting point was measuredby a method of shifting the focus and observing the triangular defectsusing an optical microscope and the surface density of the triangulardefects was obtained.

The surface density of the triangular defects in the SiC epitaxial waferof the second production lot was 0.5 pieces/cm² and the surface densityof the triangular defects having the piece of the member which is placedin the chamber as the starting point was 0 pieces/cm².

The surface density of the triangular defects in the SiC epitaxial waferof the eightieth production lot was 2 pieces/cm² and the surface densityof the triangular defects having the piece of the member which is placedin the chamber as the starting point was 0.5 pieces/cm².

The surface density of the triangular defects in the SiC epitaxial waferof the twentieth production lot was 1 pieces/cm² and the surface densityof the triangular defects having the piece of the member which is placedin the chamber as the starting point was 0 pieces/cm².

FIG. 21C shows the candela image of a production lot which ismanufactured immediately after the eightieth SiC epitaxial wafer ismanufactured and the shielding plate is replaced with a new one. Thesurface density of the triangular defects was 0.5 pieces/cm² and thesurface density of the triangular defects having the piece of the memberwhich is placed in the chamber as the starting point was 0 pieces/cm².

This result proved that, when the shielding plate was replaced with anew one, it was possible to reduce the surface density of the triangulardefects having the piece of the member which is placed in the chamber asthe starting point.

In the production lot in which the surface density of the triangulardefects having the piece of the member which is placed in the SiC-CVDfurnace as the starting point in the SiC epitaxial layer is 0.5pieces/cm², when the shielding plate is replaced with a new one, it ispossible to manufacture a SiC epitaxial wafer in which the surfacedensity of the triangular defects is 0.5 pieces/cm². When the shieldingplate is replaced with a new one in the stage in which the number ofproduction lots is less than the above-mentioned number of productionlots, it is possible to manufacture a SiC epitaxial wafer in which thesurface density of the triangular defects is 0.3 pieces/cm². Inaddition, when the shielding plate is replaced with a new one in thestage in which the number of production lots is less than theabove-mentioned number of production lots, it is possible to manufacturea SiC epitaxial wafer in which the surface density of the triangulardefects is 0.1 pieces/cm². Furthermore, when the shielding plate isreplaced with a new one in the stage in which the number of productionlots is less than the above-mentioned number of production lots, it ispossible to manufacture a SiC epitaxial wafer in which the surfacedensity of the triangular defects is 0.05 pieces/cm².

When the number of production lots in which the shielding plate isreplaced with a new one is appropriately selected, it is possible tomanufacture a SiC epitaxial wafer in which the surface density of thetriangular defects having the piece of the member which is placed in theSiC-CVD furnace as the starting point in the SiC epitaxial layer isequal to or greater than 0 pieces/cm² and equal to or less than 0.5pieces/cm². Similarly, it is possible to manufacture a SiC epitaxialwafer in which the surface density of the triangular defects is equal toor greater than 0.05 pieces/cm² and equal to or less than 0.5pieces/cm². Similarly, it is possible to manufacture a SiC epitaxialwafer in which the surface density of the triangular defects is equal toor greater than 0.05 pieces/cm² and equal to or less than 0.3pieces/cm². Similarly, it is possible to manufacture a SiC epitaxialwafer in which the surface density of the triangular defects is equal toor greater than 0.05 pieces/cm² and equal to or less than 0.1pieces/cm².

Comparative Example 1

In Comparative Example 1, manufacturing conditions were the same asthose in the example except that a shielding plate obtained by coating agraphite base with a tantalum carbide film was used in the SiC-CVDfurnace used in the example.

A SiC epitaxial wafer was repeatedly manufactured under these conditionswithout replacing the member which is placed in the chamber. FIG. 22Ashows the candela image of a twentieth production lot.

The surface density of all kinds of triangular defects in the SiCepitaxial wafer was 2 pieces/cm² and the surface density of thetriangular defects having the piece of the member which is placed in thechamber as the starting point was 1 pieces/cm².

Even though the number of repetitions of the SiC epitaxial wafermanufacturing process is the same as that in the example, both thesurface density of all kinds of triangular defects and the surfacedensity of the triangular defects having the piece of the member whichis placed in the chamber as the starting point were higher than those inthe example.

This result proved that, when the shielding plate having the graphitebase coated with the silicon carbide film was used, it was possible toreduce both the surface density of all kinds of triangular defects andthe surface density of the triangular defects having the piece of themember which is placed in the chamber as the starting point, as comparedto when the shielding plate having the graphite base coated with thetantalum carbide was used.

Comparative Example 2

In Comparative Example 2, the manufacturing conditions were the same asthose in the example except that the shielding plate was not used in theSiC-CVD furnace used in the example.

A SiC epitaxial wafer was repeatedly manufactured under these conditionswithout replacing the member which is placed in the chamber. FIG. 22Bshows the candela image of a twentieth production lot.

The surface density of all kinds of triangular defects in the SiCepitaxial wafer was 100 pieces/cm² and the surface density of thetriangular defects having a piece of the member which is placed in thechamber as a starting point was 90 pieces/cm².

Even though the number of repetitions of the SiC epitaxial wafermanufacturing process is the same as that in the example, both thesurface density of all kinds of triangular defects and the surfacedensity of the triangular defects having the piece of the member whichis placed in the chamber as the starting point were significantly higherthan those in the example.

This result proved that the use of the shielding plate made it possibleto significantly reduce the surface density of all kinds of triangulardefects and the surface density of the triangular defects having thepiece of the member which is placed in the chamber as the startingpoint.

INDUSTRIAL APPLICABILITY

It is possible to provide a method of manufacturing a SiC epitaxialwafer with a low surface density of triangular defects which haveparticles attached to a SiC substrate as starting points.

REFERENCE SIGNS LIST

-   -   1: CHAMBER (SIC-CVD FURNACE)    -   1 a: INNER WALL    -   2: SUSCEPTOR    -   2 b: MOUNTING PORTION    -   3: CEILING    -   4: REACTION SPACE    -   6, 7: INDUCTION COIL (HEATING MEANS)    -   10: SHIELDING PLATE    -   10 a: OUTER CIRCUMFERENTIAL PORTION    -   10A, 10B: SHIELDING PLATE    -   11: SUPPORTING PORTION

1. A SiC epitaxial wafer comprising: a SiC epitaxial layer that is formed on a SiC substrate having an off angle, wherein the surface density of triangular defects, in which a distance from a starting point to an opposite side in a horizontal direction is equal to or greater than (a thickness of the SiC epitaxial layer/tan(x))×90% and equal to or less than (the thickness of the SiC epitaxial layer/tan(x))×110%, in the SiC epitaxial layer is in the range of 0.05 pieces/cm² to 0.5 pieces/cm² (where x indicates the off angle).
 2. The SiC epitaxial wafer according to claim 1, wherein the surface density of triangular defects with a starting point size of less than 20 μm is equal to or less than 0.05 pieces/cm².
 3. The SiC epitaxial wafer according to claim 1, wherein the surface density of triangular defects including a piece of a member which is placed in a SiC-CVD furnace as a starting point in the SiC epitaxial layer is equal to or less than 0.5 pieces/cm².
 4. The SiC epitaxial wafer according to claim 3, wherein the piece of the member as the starting point is made of one of carbon and silicon carbide. 